This invention relates generally to rotary machines and, more particularly, to fuel recirculation systems and nitrogen purge systems.
In some known dual-fuel combustion turbines, the turbine is powered by burning either a gaseous fuel or a liquid fuel, the latter fuel typically being distillate oil. These combustion turbines have fuel supply systems for both liquid and gas fuels. Combustion turbines generally do not burn both gas and liquid fuels at the same time. Rather, when the combustion turbine burns liquid fuel, the gas fuel supply is removed from service. Alternatively, when the combustion turbine burns gaseous fuel, the liquid fuel supply is removed from service.
In some known industrial combustion turbines, a combustion system may have an array of combustion cans, each of which has at least one liquid fuel nozzle and at least one gas fuel nozzle. In the combustion can arrangement, combustion is initiated within the combustion cans at a point slightly downstream of the nozzles. Air from the compressor (normally used to deliver compressed air to the combustion system) flows around and through the combustion cans to provide oxygen for combustion.
Some known existing combustion turbines that have dual fuel capacity (gas fuel as primary and liquid fuel as backup) may be susceptible to carbon deposits, in the form of carbonaceous precipitate particulates, forming in the liquid fuel system. Carbonaceous particulate precipitation and subsequent deposition generally begins when liquid fuel is heated to a temperature of 177° C. (350° F.) in the absence of oxygen. In the presence of oxygen, the process accelerates and carbonaceous particulate precipitation begins at approximately 93° C. (200° F.). As carbonaceous particulates accumulate, they effectively reduce the cross-sectional passages through which the liquid fuel flows. If the carbonaceous particulate precipitation continues unabated, particulates may obstruct the liquid fuel passages. In general, the warmer areas of a combustion turbine tend to be associated with the combustion system that is located in the turbine compartment of many known combustion turbine systems. Therefore, the formation of carbonaceous particulates will most likely be facilitated when subjected to the turbine compartment's heat and may not be present in the liquid fuel system upstream of the turbine compartment.
Prior to burning gas fuel the liquid fuel nozzle passages are normally purged via a purge air system that is flow connected to the liquid fuel system. However, static liquid fuel may remain in a portion of the system positioned in the turbine compartment to facilitate readiness for a rapid fuel transfer. During those periods when the liquid fuel system is removed from service, the purge air system is at a higher pressure at the point of flow communication with the liquid fuel system and air infiltration into a portion of the liquid fuel system is more likely. This condition may increase the potential for interaction between fuel and air and, subsequently, carbonaceous particulate formation may be facilitated.
In general, when liquid fuel systems remain out of service beyond a predetermined time limit, there is an increased likelihood that the static liquid fuel within the turbine compartment will begin to experience carbonaceous particulate precipitation. Purge air infiltration into the liquid fuel system facilitates air contact with liquid fuel and the potential for extended air-to-fuel interaction increases as the length of period of time associated with maintaining the fuel system out of service increases and the magnitude of air infiltration increases. As noted above, liquid fuel carbonaceous particulate precipitation is facilitated at a much lower temperature in the presence of oxygen. Considering that some known turbine compartment temperatures have been measured in excess of 157° C. (315° F.), carbonaceous particulate precipitation is even more likely to occur if infiltrating purge air remains in contact with static liquid fuel. As carbonaceous particulates form, they pose the potential of obstructing liquid fuel internal flow passages, including those in the combustion fuel nozzles.